Magnetic induction accelerator



Sept. 29, 1942. p w KERsT 2,297,305

MAGNETIC INDUCT ION ACCELERATOR Filed Nov. 13, 1940 2 Shees-Sheet 1 Inventor: Donald W.Kerst,

b flamyrfi His Attorney.

Sept. 29, 1942. w KERST 2,297,305

MAGNETIC INDUCTION ACCELERATOR Filed NOV. 13, 1940 2 Sheets-Sheet 2 Fig.5.

7PM4GIV7 60/4 5 l 4 7 lg.

Inventor:

Dohald W. Kerst,

y Hl's Attorney.

Patented Se t. 29, 1942 MAGNETIC INDUCTION ACCELERATOR Donald W. Kerst, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application November 13, 1940, Serial No. 365,520

7 Claims.

The present invention relates to apparatus for accelerating charged particles, such as electrons, by means of magnetic induction effects.

It has previously been proposed to obtain high velocity electrons by the use of a closed vessel defining an annular path for electron gyration and a magnetic system for producing a timevarying magnetic field of such space distribution as to confine electrons projected within the vessel to a circular orbit along which they are continuously accelerated by the field. However, the forms of such apparatus which have heretofore been described have been either inoperable or operable only in an extremely limited sense. It is an object of the present invention to provide an improved magnetic accelerator of the circular orbit type which is capable of realizing a substantial output of electrons (or other charged particles) of very high velocity.

In the attainment of the foregoing object an important feature of the invention consists in the provision of improved means for introducing charged particles into the orbital path in which acceleration is to occur. In particular, it is proposed in this connection to generate such p ticles within the region of influence of the magnetic accelerating field and to project them with an initial velocity calculated to assure their capture by the field-producing system employed.

Another important feature of the invention, ancillary to the above, consists in the provision of means for continuously varying the velocity of the injected particles in a manner correlated to the rate of variation of the magnetic accelerating field. This increases the length of the period during which electrons may be captured by the magnetic field and thus leads to an increase in the output of the accelerating apparatus as a whole.

A still further important feature of the invention comprises the inclusion in connection with the acceleration vessel of means for regularizing the electric field distribution around the orbital path of the charged particles and for guarding the particles against displacement from such path by electrostatic causes.

The aspects of the invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the drawings in which Fig. 1 is a sectional elevation of an accelerating apparatus suitably embodying the invention; Fig. 2 is a cross-section taken on line 2-2 of Fig. 1; Fig. 3 represents an enlarged fragmentary portion of the apparatus of Fig. 1; Fig. 3a is a fragmentary view showing a possible modification of one feature of the invention; Fig. 4 is a sectional view of a modified form of discharge device adapted for use in connection with the invention; Fig. 5 shows diagrammatically an energizing circuit which may be employed in one mode of use of the invention; Figs. 6 and 7 show structural variations of the pole pieces of the apparatus of Fig. 1; Fig. 8 illustrates diagrammatically one method of energizing the magnetic system, and Fig. 9 shows an alternative arrangement for the same purpose.

Referring particularly to Fig. 1 there is shown in section a closed glass vessel I0 which defines Within its interior a continuous annular chamber H. As will be explained in greater detail at a later point, the vessel I0 provides a circular orbit in which electrons may be accelerated to a high voltage, say, on the order of several million volts. The vessel is preferably highly evacuated, although the presence of a readily ionizable gas at a pressure not in excess of 10- mm. of mercury has some advantages with respect to the neutralization of space charge.

The accelerating mechanism comprises a mag netic structure having generally circular pole pieces which are coaxial with the annular vessel I 0. These pole pieces include a pair of imitaposed circular parts [3 and I4 which consist, for example, of compressed powdered iron and which are respectively supported on conically tapered parts and i6. The tapered parts in turn are based upon large cylinders l8 and [9 which connect with closed magnetic cores 2| and 22 so as to provide a complete path for magnetic flux. The magnetic structure is energized by means of a pair of serially connected coils 24, 25 which are appropriately mounted on the structure. It is assumed that the coils are excited from an alternating current source or in some other manner adapted to produce a time-varying flux in the magnetic circuit. The elements of the magnetic structure are, of course, constituted of ferromagnetic material and should be of laminated or otherwise subdivided construction, so as to avoid the generation of excessive eddy currents.

Within the closed vessel [0 and also within the region of influence of the magnetic field produced by the pole pieces l5 and I 6 there is provided a. thermionic cathode 28 which, in conjunction with other electrode structure to be later described, serves to generate a stream of electrons. These electrons are affected by the magnetic field in two ways. In the first place, since the field is in a direction transverse to the plane of the electron motion, it tends to force the electrons to follow a generally circular orbit. Secondly, the time-varying flux inclosed by the orbit of any particular electron necessarily produces an accelerating action on the electron. In this latter respect, the apparatus as a whole consists essentially of a transformer with a singleturn secondary comprising a circular path along which the various electrons are accelerated. Although, in general, the voltage per turn in such a transformer is low, the electrons can achieve very high velocities (e. g. several million volts) because of the tremendous number of turns which they may execute during a single cycle of the field variation.

It has been shown (see, for example, Steenbeck Patent 2,103,303, granted December 28, 193'?) that by a proper design of the magnetic structure the field existing at the electron orbit may be caused to produce a centripetal force in balance with the centrifugal tendencies of the accelerated electrons. In general, this result requires that the following relationship be satisfied:

where is the fiux included'wlthin the electron orbit, r is the radius of the electron orbit, and Hr is the field strength at the orbit. This equation obviously means that the flux must be twice as strong as that which would be produced by a homogeneous field equal to the field Hr extending over the entire area enclosed by the orbital electron path,

The condition just specified may be realized by making the reluctance of the magnetic path greater by an appropriate amount at the electron orbit than its average reluctance within the orbit. In order to maintain fixed proportionality between the enclosed flux and the guide field (i. e. the field Hr) at all times during the accelerating period, one may include in the magnetic path an air gap or its equivalent. It is readily practicable to adjust the dimensions of such a gap from point to point over the pole area in such a fashion as to effect the balanced relationship of guide field and enclosed fiux which is desired for the purposes specified above.

The balanced relationship just referred to represents a condition of stable operation for electrons following the ideal orbit, but does not take into consideration the requirements of electrons which tend for one reason or another to deviate from such an orbit. For example, deviation of a particular electron in either the radial or the axial direction may occur as a result of collision of the electron with a gas molecule or as a result of space charge effects; the field relationship established by equation (1) does not guarantee the restoration of such an electron to its desired course. It is found that axial stability may be obtained by fulfillment of the condition that the space rate of variation of the magnetic field in the radial direction shall be negative; that is, that the field shall continuously decrease as one proceeds radially outward. On the other hand, radial stability requires fulfillment of the condition that 14E H dr shall not be more negative than -1. A satisfactory compromise between the conditions for axial and radial stability may be obtained by making H proportional to where n is between zero and 1. A relatively small value of n, within these limits, causes little axial focusing and relatively large radial focusing, while a large value of n gives the converse effect. A suitable value of n is found to be 2/3, this being a value which is substantially realized it the magnetic pole pieces are formed in the manner indicated in connection with parts l5 and I6 of Fig. 1.

The eilfect of the focusing fields referred to in the foregoing may be understood by considering the case of a particular electron which is in some way displaced from the ideal orbit. If such an electron is displaced in an axial direction, the axial focusing field will urge the electron back toward the ideal orbit, causing it to swing back and forth across the orbit with a sort of hunting motion. A precisely similar action will occur if the electron suffers a radial displacement or a displacement which is partially radial-and partially axial. When the magnetic field is increasing with time (as in the case under consideration), the amplitude of the electron oscillation about its ideal orbit decreases on successive swings so that there is a tendency for the electron eventually to be restored to the ideal orbit and for the electron stream as a whole to be focused in such orbit.

'A principal problem in the operation of apparatus of the type under consideration consists in the difficulty experienced in introducing electrons into the accelerator. In general, electrons projected into a magnetic field from an external source tend to be rejected by the field and will be captured by it only if special conditions exist. I have found it possible to a large extent to over come the problems of electron injection by providing an electron source which is actually within the region of influence of the magnetic field (specifically within the region in which the field obeys the relation metry of the vessel Hi. This cathode is enclosed laterally by a pair of metal electrode elements 29 which are arranged in juxtaposed relation to provide at their edges a pair of slots which permit the escape of electrons emitted by the cathode in a. direction generally tangential to the outer wall of the vessel Hi. The members 29 are in turn enclosed by a somewhat similar pair of conductive elements 30 positioned to provide slots in alignment with the slots provided between the members 29. In the operation of the electron injector, the electrode elements 29 are normally maintained at a potential which is negative with respect to the cathode 26, and the elements 30 are maintained at a relatively positive potential.

This may be done, for example, by connecting the electrode elements to an external potential source 3|, as illustrated in Fig. 1. With this arrangement of electrodes, which is shown in somewhat greater detail in Fig. 3, the resultant electrostatic fields tend to produce a thin ribbon of electrons proceeding in both directions through the slots provided between the various electrode elements. With a given direction of the magnetic field, only the electrons projected from one side of the electrode system are utilized, the remainder (that is, those projected in the other direction) being deflected away from the accelerating orbit against the outer wall of the enclosing vessel.

The electron injector may, if desired, be positioned near the inner rather than the outer peripherybf the accelerating chamber. This alternative construction is illustrated in Fig. 4, which shows electrode elements 28', 29 and II in an appropriate arrangement.

The velocity imparted to the electrons by the positively charged electrode elements 30 (or 30' in Fig. 4) should be high enough so that scattering due to space charge and to the presence of residual gas in the vessel I is not excessive, and in this connection an electron energy of at least 30 to 40 electron volts is necessary. upper limit on the electron velocity is imposed by the condition that the inward displacement .of a given electron produced by the changing magnetic field during the initial orbital transit of the electron must be great enough to prevent the electron from being intercepted by the injector electrodes after the completion of such transit. In addition, it is necessary that the effect of the increasing magnetic field in damping the back-and-forth oscillations of the electron as it seeks a stable orbit shall be great enough to prevent these oscillations from leading to interception of the electron. This damping efiect and the inward displacement efiect above referred to are both inversely related to the electron Velocity and therefore require some limitation of the latter parameter. In general, experimentation has shown that the attainment of satisfactory results requires that the electrons be injected with an energy of at least several hundred electron volts and has shown further that energies as high as several thousand electron volts may be employed without the occurrence of excessive electron interception.

As a further aid with respect to the problem of avoiding interception of the electrons by the injector electrodes, it is advantageous in some cases to taper the pole pieces i5 and it somewhat more steeply than is indicated in Fig. 1 so as to cause the space distribution of the magnetic field to vary inversely as the three-fourths power of the radial distance from the center of the magnetic structure. With this arrangement, it can be shown that electrons which leave the injector along more or less divergent paths are brought to a sharp focus at the end of each orbital transit. These electrons have a better chance of missing the injector than electrons released under conditions which do not produce such focusing.

In order to assure the capture of injected electrons, there must be a definite correlation between the velocity of the electrons and the strength of the magnetic field at the time of injection. In the case in which fixed potentials are applied to the electrode parts 29 and 30 so as to produce electrons of a single velocity, the produced electrons will be accepted by the magnetic system only during the brief interval when the guide field at some point within the chamber H has a value calculated to balance the centrifugal tendencies of electrons of the particular velocity involved. The number of electrons which may be captured with this mode of operation is therefore relatively limited-although not so much so as to preclude successful functioning oi the apparatus.

As an alternative to this procedure, one may connect the electrode structures, and particularly the accelerating electrode parts 30, to a source of cyclically varying potential which is properly synchronized with the exciting source for the magnetic field system. With this arrangement, which obviously results in the pro'-' duction of electrons of differing initial velocities, electrons may be caused to be captured by the magnetic field for as long a period as the proper correlation between electron velocity and magnetic field strength can be maintained.

The method of electron injection last described requires that the voltage on the accelerating electrode shall increase about as the square of the time, counting zero time as that at which the magnetic field is zero. Thi condition may be fulfilled in one way by a circuit of the character shown in Fig. 5 in which the injector electrodes 28, 29 and 30 are indicated diagrammatically. The circuit is triggered through a so-called peaking transformer" MI which possesses a readily saturable magnetic circuit of such character as to facilitate the production of sharp pulses of voltage in the transformer secondary H. (The theory and construction of transformers of this type are fully described in B. D. Bedford Patent No. 1,918,173, granted July 11, 1933.) The primary winding 42 of the peaking transformer is assumed to be connected in series with the magnet coils 24 and 25 of the apparatus of Fig. 1, so that the pulses of voltage induced in the secondary winding fll are synchronized with the variations of the field by which the electrons within the vessel H) are accelerated. A saturating winding 43, excited from a direct current source M, permits the phase relationship between the voltage pulses developed in the winding 4| and the variations of the magnetic held to be adjusted within close limits. With the adjustment which is preferred for present purposes, a voltage pulse is generated at the beginning of each rising cycle of the magnetic field.

The pulses thus produced are impressed on the grid 45 of a high vacuum discharge tube 46. The tube 46 includes, in addition to the grid 45, a cathode M and an anode 48, the latter being maintained at an appropriate positive potential. An inductance 50 and a condenser 5| are connected serially in the cathode circuit, and a further condenser 52 is connected between the anode and ground.

As soon as a pulse of positive potential is impressed on the grid 45 (i. e. from the winding M) the tube 46 is rendered conductive, and the energy stored in the condenser 52 is permitted to flow into the condenser 5| through the inductance 50. Assuming an appropriate condition of resonance of the elements 50 and 5|, the voltage across the latter builds up approximately as the square of the elapsed time, at least for a brief initial period. This voltage is impressed on the injector electrodes 28, 29 and 30 by means of a voltage divider 53, this being connected to the electrodes as shown. The velocity of the injected trostatic forces can readily deflect the electrons.

The elimination of improper electrostatic fields, which are apt to arise, for instance, from electrons hitting the walls of the vessel [0, may be accomplished by providing a uniformly conducting lining on the interior surface of the accelerator chamber. Such a lining may consist, for example, of a very thin silver coating 32 applied by evaporation or otherwise to the chamber surface. (See Fig. 3.) The coating must be of sufficiently high resistance to prevent the circulation of eddy currents of such magnitude as to cause excessive heating and is preferably maintained at the same potential as the electrodes 30 (Fig. 1) so as to avoid production within the accelerating chamber of transverse fields which might tend to cause lateral deflection of the electron beam. (In some cases, it is advantageous to maintain the coating somewhat positive with respect to the injector structure in order to establish fields tending to cause orbitally moving electrons to miss this structure.) Such a coating tends to regularize the electric field distribution around the electron orbit and to assure the uniform acceleration of electrons along such orbit,

Another method of avoiding unwanted electrostatic fields within the acceleration chamber is to coil a wire helically around the beam path so that it substantially covers the entire interior or exterior surface of the enclosing vessel. This modification is illustrated in Fig. 3a which shows a fragmentary section of an annular vessel Ill" having a wire 33 wrapped around its external surface. Current passed through the wire 33 from a direct current source 34 produces a ma netic field which is directed along the electron orbitand which tends to maintain the electron beam in focused condition. Additional focusing may be obtained by varying the distribution of the wire coil as by concentrating it at certain points to produce magnetic lens effects.

It is found most expedient to utilize the energy of the accelerated electrons indirectly by causing them to impinge on a scattering body such as a target which is arranged to be struck by the beam after it has attained its ultimate speed. Such a target is indicated at 35 in Figs. 1 and 2 where it is shown as comprising a wedge-shaped metal piece having surfaces arranged to be struck by electrons proceeding in either direction around the annular orbit. The target 35 is preferably connected to the conductive coating 32 which lines the interior of the tube envelope (as indicated at 36 in Fig. 1) and is desirably constituted of a metal such as tungsten or lead. With metals of the latter class, electrons impinging on the target are in part reflected, while the remainder generate X-rays in a known manner. Either the refiected electrons or the X-rays or both may be utilized, depending upon the desired application of the apparatus. In any case, it is the function of the target to make the electron energy available in some form outside the acceleration chamber.

In [order to cause the electron stream to impinge upon the target 35 it is, of course, necessary that the electrons be shifted from the orbit in which their acceleration occurs. done, for example, by destroying the balance between the accelerating field and the guide field as the end of the accelerating cycle is approached. (It is obvious'that if the enclosed flux becomes too great or too small for a balanced condition to exist, the electrons will be diverted outwardly or inwardly as the case may be.) In order to produce such an unbalance one may employ saturation effects caused, for example, by including a saturable portion in the magnetic structure. This may be done in one way by the arrangement shown in Fig. 6 which illustrates pole pieces l4 and I6 corresponding in essential particulars to the parts l4 and IS in Fig. 1.. The pole piece I4 is provided at its central area with an insert 38 consisting, for example, of a quantity of iron powder embedied in a suitable non-magnetic binder such as an insulating plastic. The composition of this insert is made such as to cause its iron components to become saturated as the magnetic flux approaches its maximum value. With this arrangement, the aforesaid saturation produces a condition of insuflicient enclosed flux and results in an inward deflection of the electron stream which, in the apparatus of Fig. 1, permits it to impinge on the target 35.

A generally similar result can be obtained in I a different way by the arrangement illustrated in Fig. '7, in which the pole piece I4" is provided with an inset conducting ring 40 (shown in section). This ring provides a closed loop for the circulation of eddy currents and therefore acts as a shading coil which causes the fiux linked by the ring to lag behind the magnetic field. Consequently, when the magnetic field is at a maximum, the fiux through the shading coil continues to increase, thus destroying the proportionality between field and flux. The direction of this change is such as to cause the electron path to expand outwardly, and this arrangement is therefore mainly applicable in connection with a system in which the electrons are generated at a point near the inner periphery of the annular chamber and are to be intercepted at a point near its outer periphery. This is the case illustrated in Fig. 4, in which the target element is shown at 35.

,Up to this point, no consideration has been given to the particular means employed for producing a changing field in the magnetic structure. Such a changing field may be produced in one way by an arrangement such as is shown in Fig. 8, which illustrates diagrammatically a portion of the structure of Fig. 1. The coils 24 and 25 are shown connected in parallel with each other and with a bank of condensers 52 which are assumed to be of such capacity as to resonate with the inductance of the coils at a frequency corresponding to the desired frequency of operation of the apparatus. This may be, for example, on the order of 600 cycles per second, although frequencies differing widely from this value are also usable. To supply the losses of the resonant circuit. thus formed, the coils 24 and 25 may be coupled to single turn coils 53 and 54 which are directly energized from a high frequency (e. g. 600 cycle) generator 56. A relatively small amount of power supplied by the generator will serve to maintain the resonant system in excited condition.

This may be flux of the desired character is to energize a portion of the magnetic circuit with direct current and to rotate this portion in such manner that it produces an intermittently reversing flux in the remainder of the circuit. Such an arrangement is illustrated in Fig. 9 which shows a magnetic system having the same general function as that of Fig. 1. This has pole pieces 60 and 6| and a core structure which is indicated as a whole by the numeral 62. An intermediate portion 63 of the core structure is mounted so as to be independently rotatable and is provided with a direct current winding 66 which is energized from an appropriate source 61. By driving the rotatable part 63 through the agency of a motor 68, the flux which it produces through the poles is obviously caused to change in periodic fashion. The frequency of reversal of the magnetic field must be such that the rate of change during the period when electrons are being injected is sufficiently high to make the electron beam miss the injector electrodes on its first and successive orbital transits.

While the invention has been described by reference to particular embodiments thereof, it will be understood that numerous changes may be made without actually departing from the invention. In particular, although the description given has been concerned mainly with the acceleration of electrons, it is obvious that the invention is equally applicable in connection with other charged particles, such as positive or negative ions. I therefore aim in the appended claims to cover this and all such equivalent variations of application and structure as are within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a magnetic induction accelerator for charged particles; a closed vessel defining a chamber within which such particles may move in a circular path, a magnetic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with the vessel, means for producing a time-varying magnetic field between said pole pieces, the pole pieces being of outwardly tapered configuration such that the magnetic field H varies inversely with the radial displacement 1 according to the proportionality Ha T where n is between zero and unity, and means including a source of charged particles within the said annular chamber and also within the region in which the magnetic field distribution follows the aforementioned proportionality for projecting charged particles within the annular chamber in a definite direction and with a substantial initial velocity.

2. In a magnetic induction electron accelerator; a closed vessel defining a chamber within which electrons may move in a circular path, a magnetic structure outside said vessel and comprising opposed generally circular pole pieces which are coaxial with the vessel, means for producing a time-varying magnetic field between the pole pieces, the pole pieces being of outwardly tapered configuration such that the magnetic field H varies inversely with the radial displacement 1' according to the proportionality Har where n is-between zero and unity, and electrongenerating means within said chamber and also within the region within which the magnetic field distribution follows the aforesaid proportionality, said last-named means comprising a cathode and electrodes cooperating therewith for projecting within the annular chamber a directed stream of electrons having an initial energy of at least several electron volts.

3. In combination, a closed vessel defining a rotationally symmetrical chamber, means within the chamber and in proximity to a lateral wall thereof for generating charged particles, electrode structure for initially accelerating charged particles produced by said last-named means in a direction at least approximately tangential to the said chamber wall, a magnetic structure outside the said vessel for producing a magnetic field which increases with time, said field being generally parallel to the axis of symmetry of said chamber and of such space distribution as to confine the said charged particles to a circular orbit while continuously accelerating them along said orbit, and means for applying to the said accelerating electrode structure a voltage which increases with time in a manner correlated to the rate of increase of the said magnetic field.

4. In combination, a closed vessel defining an annular chamber, means within the chamber and in proximity to a lateral wall thereof for generating charged particles, electrode structure for initially accelerating charged particles generated by said means in a direction at least approximately tangential to the chamber wall, means including a magnetic structure outside the said vessel for producing a magnetic field which increases sinusoidally with time, said field being generally parallel to the axis of symmetry of said chamber and of such space distribution as to confine the said charged particles ,to a circular orbit while continuously accelerating them along said orbit, and means for applying to the said accelerating electrode structure an accelerating voltage which varies as the square of the elapsed time whereby a continuous correlation is maintained between the velocity of the charged particles accelerated by the electrode structure and the strength of the magnetic field.

5. In amagnetic induction electron accelerator; a closed vessel defining a chamber within which electrons may move in a circular path, a magnetic structure outside said vessel and comprising opposed generally circular pole pieces which are co axial with the vessel, means for producing a timevarying magnetic field between said pole pieces, the pole pieces being of outwardly tapered configuration such that the magnetic field H varies inversely with the radial displacement according to the proportionality Her- T" to high velocity by the action of the magnetic 6. In a magnetic induction electron acceleratorf a closed vessel defining an annular chamber, a magnetic structure outside said vessel and comprising opposed,- generally circular pole pieces which are coaxial with the vessel, means for producing a time-varying magnetic field between said pole pieces, the pole pieces being of outwardly tapered configuration such that the magnetic field H varies inversely with the radial displacement 1- according to the proportionality where n is between zero and unity, electrode structure within the said annular chamber and also within the region in which the magnetic field distribution follows the aforementioned proportionality for projecting electrons within said anfor electron gyration and means producing a time-varying magnetic field which acts transversely to the plane of the said path and which has a space distribution calculated to maintain electrons following the path in a confined orbit while the electrons are being continuously accelerated by the field, the improvement which consists in the combination of electrode structure within the closed vessel and also within the region or influence oi the said magnetic field for projecting a directed stream of electrons into the said annular path, and means for regularizing the distribution of electric field around the said annular path, said last-named means comprising a wire helically surrounding the said annular path for shielding the same, and means for passing current through the wire to produce a magnetic field tending to maintain the electron stream in focused condition.

DONALD W. KERST. 

